1. Field of the Invention
The present invention relates in general to an asphalt finisher for road paving, and related in particular to an automatic control system for use with the asphalt finisher.
2. Technical Background
Automation is a key factor in efficient and economical road construction, and much effort is expended in developing automated control systems for various aspects of asphalt finishers. For example, the inventors of the present invention have previously disclosed an automatic directional control device for use with an asphalt finisher (Japanese Patent Application, Second Publication, H4-32883), and a device for controlling the operation of screeds of an asphalt finisher (Japanese Utility Model Application No. H3-34781).
The automatic directional control device (H4-32883) was designed to detect the travel direction of an asphalt finisher (referred to as the finisher herein below) in accordance with the signals emitted from three light sensors which are arranged laterally on the front section of a finisher body to detect a reference line disposed on a roadside.
The screed is a device for levelling the asphalt within a defined region of the road so as to align appropriately to the edges of the road, and is disposed on the side sections of the finisher. The screeds are required to be extended or contracted depending on the position of the finisher with respect to the reference line. In a conventional finisher, this operation is performed manually by a finisher operator who controls fluid pressure in a plurality of hydraulic cylinders so as to move the cylinders to left or right in accordance with his visual confirmation of the reference line and instructions from a screed manager. The screed manager is responsible for the quality of the paved road, and he may be required to travel from side to side to check the direction of the travel or he may choose to assign the task of monitoring the road width to a couple of sidemen to watch the edges of the laid down asphalt, so that he may concentrate on the duty of maintaining the paving quality. Such working conditions are generally not satisfactory and ultimately resulted in manual adjustments of the road width, and other such labor-intensive corrective measures.
An improvement in the conventional approach was achieved in the above-noted invention (H3-34781), by providing a detector, such as a CCD camera to detect the reference line, so as to controller the expansion or retraction of the screed by a screed control device.
This screed controller device described above presented the following problem, however. When the detector is disposed at the rear section of the finisher, and if the road width is becoming narrow, the front edge of the screed projects outward of the road and results in laying down of the asphalt mixture beyond the reference line. This type of design is also not applicable when the reference line is based on roadside objects which project out of the ground surface, because the screed tended to collide with it.
For an efficient operation of the finisher, it is ideal to provide an integrated automatic control of the finisher which is capable of accommodating changing widths of the road. However, in developing a control system for these devices described above, it was found that two sets of sensors, one for directional control and one for screed control, be placed at different location of the finisher. This presented a problem that the system became complex and resulted in a high cost for the control system.